This is a good 15 minutes of searching:http://www.monarchlab.umn.edu/lab/resea ... easey.aspx
6th paragraph down:
"A study by Etchegaray et al. (1975) showed tachinid flies prefer to lay eggs on late second-, third-, or early fourth-instar monarch larvae and are usually unable to lay eggs on fifth-instars because these instars are able to shake off fly eggs. Although unusual, Etchegaray found that tachinid flies were able to lay eggs on first-instar monarch larvae. Etchegaray also observed superparasitism when 20 or more tachinid eggs were laid on a single monarch larva; the monarch larva and tachinid larvae died as a result (1)."http://www.monarchwatch.org/update/2003/tachinid.html
Large pictures of the fly pupae and flyhttp://www.monarchwatch.org/update/2003/0717.html#8
This update tells about the symptoms of a larvae that has been parasitized.http://www.mymonarchguide.com/2008/01/d ... d-fly.html
The Dreaded Tachinid Fly http://www.mlmp.org/Results/Findings/Default.aspx
There's a lot of good research here.http://www.mlmp.org/Results/Findings/Ob ... m_2007.pdf
Parasitism of Monarch Butterflies (Danaus plexippus) by
Lespesia archippivora (Diptera: Tachinidae)
"Three flies emerged from monarchs collected as eggs in her MI Sites; these were
identified as Lespesia archippivora."
"The odds ratios indicate that the likelihood of being parasitized roughly doubles for successive stadia."
Mona's Note: 3rd and 4th instar are more susceptible. Apparently, 5th may be able to withstand an attach and keep the fly from laying.